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At least 1,117 records · Page 62Linked to original sources
PubMed2003

Electrophysiological correlates of purely temporal figure-ground segregation.

Inhomogenous displays, in contrast to homogenous ones, evoke a specific potential in the VEP (tsVEP) which appears across different classical visual stimulus dimensions defining figure-ground segregation, such as luminance, orientation, (first-order) motion, and stereoscopic depth. This negative potential has a peak latency of about 200-300 ms and a peak amplitude of about -3 to -10 microV [Doc Ophthalmol. 95 (1998) 335]. Previously, we demonstrated that human subjects reliably segregate figure from ground, even in the absence of the classical cues, leaving time of change as the only cue for segregation. The results of the present study demonstrate that also purely temporally defined checkerboards evoke a tsVEP resembling the motion-defined tsVEP regarding polarity (negative), latency (two peaks at 180 and 270 ms, respectively), amplitude of the first negativity (-5.6 microV), and overall form of its components.

Adult↗
PubMed1997

Velocity decomposition and surface decomposition--reciprocal interactions between motion and form processing.

Physically unidirectional motion of short-lived random dot arrays was found to perceptually decompose into two motion components (velocity decomposition) in a configuration in which two squares appear to partially overlap transparently (surface decomposition). In the experiments in which the velocity of the short-lived random dots in the overlapping area was varied, both the velocity decomposition and the surface decomposition were found to be strongest when the velocity of the overlapping area was close to the vector sum of the velocities of random dots in adjacent non-overlapping areas. On the other hand, neither velocity decomposition nor surface decomposition was found either when random dot arrays were put in occlusion configurations or when continuous random dots were used. While previous studies have indicated a one-way influence either from motion to form processing, or from form to motion processing, the present study further suggests that there is a strong reciprocal interaction between motion and form processing. A possibility is that the reciprocal interaction is iterative so that the representations for velocity and surface decomposition are gradually formed.

Female↗
PubMed1997

Motion processing for saccadic eye movements during the visually induced sensation of ego-motion in humans.

During ego-motion an observer is often faced with the task of controlling his heading direction while simultaneously registering the movement of objects in order to avoid possible obstacles. Psychophysical experiments have shown that the detection of moving objects is impaired by concurrent ego-motion. We investigated the interaction between ego-motion and object-motion by examining the latencies of saccades executed to moving targets under a visually induced sensation of ego-motion. Saccadic latencies increased during this sensation, with a global or non-retinotopic effect of optic flow on motion detection. Furthermore, separating stereoscopically the moving target and the optic flow into foreground and background, respectively, still resulted in increased latencies. We propose that an inhibitory influence of the perception of self-motion exists on the perception of object-motion. These results support a model of space constancy which strives to create a stable world during locomotion.

Adult↗
PubMed1997

Sequential stereopsis using high-pass spatial frequency filtered textures.

Enright [(1995). Perception, 24 (suppl.), 32-33; (1996). Vision Research, 36, 307-312] described a simple piece of equipment for demonstrating a perceptual mechanism he called sequential stereopsis. The equipment requires an observer to set two textured targets seen behind a pair of small viewing ports to appear equi-distant. The principle upon which the apparatus depends is the use of textures whose elements cannot be resolved in peripheral vision at the eccentricity determined by the target separation. Enright used a fine sandpaper for this purpose. We have conducted two similar experiments using high bandpass filtered textures which eliminate any possibility that the low spatial frequency content of sandpaper textures could play a role. Our results corroborate Enright's general conclusions on sequential stereopsis, while at the same time showing that high-pass textures do not give wholly similar results to sandpaper.

Adult↗
PubMed1998

Evidence for separate control of slow version and vergence eye movements: support for Hering's Law.

When a visual stimulus changes direction and distance simultaneously, Hering's Law argues that the resulting eye movements are the result of combined version and vergence control processes. Recently, it has been suggested that slow asymmetrical eye movements might be guided by monocular control processes wherein each eye is driven by its own retinal image. Experimental results presented here show behavioral differences between slow version and slow vergence eye movements, indicating that different control processes drive the two "pure" responses. Specifically, version tracking of constant velocity stimuli (i.e., smooth pursuit) is more precise, showing less variation in tracking velocity than movements of equal velocity produced by vergence stimuli. When the two stimuli are combined, the variability in tracking is consistent with the addition of the two components in proportion to their respective stimuli. These results provide support for Hering's Law, at least for low velocity, smooth tracking movements (i.e., slow version and slow vergence.

Adult↗
PubMed1998

Optic flow and the metric of the visual ground plane.

A theory is developed in which the optic flow of an observer translating over the ground plane determines the metric of egocentric visual space. Optic flow is used to operationalize the equality of spatial intervals not unlike physicists use time to compare spatial intervals. The theory predicts empirical matching ratios for collinear, sagittal intervals to within 2% of the mean (eight subjects, standard error also 2%). The theory predicts that frontoparallel intervals on the ground plane will match sagittal intervals if their relative image motions match, which was found empirically. It is suggested that the optic flow metric serves to calibrate static depth cues such as angular elevation and binocular parallax.

Adult↗
PubMed1998

The visual effects of head-mounted display (HMD) are not distinguishable from those of desk-top computer display.

Concerns about potentially harmful effects on the visual system due to the use of head mounted displays (HMDs) in general, and stereoscopic systems in particular, have been raised in the literature. Most of the concerns were based on studies measuring visual function changes following short-term use of HMDs. This study measured functional changes in binocular vision, accommodation, and resolution following 30 min use of HMD in both stereoscopic- and non-stereoscopic modes, and compared them to changes following the same task performed on a desk-top CRT display. No functional differences were found between HMD and CRT and most measured changes were too small to be considered clinically meaningful. An evaluation of subjective comfort found a statistically significant difference in the impression of comfort between the CRT and the HMD in stereoscopic mode, with the latter being less comfortable. It can be concluded that the functional changes reported following short term use of HMDs are not specific to stereoscopic presentation and do not differ from those caused by desk-top CRT display.

Accommodation, Ocular↗
PubMed1999

Perceived distance, shape and size.

If distance, shape and size are judged independently from the retinal and extra-retinal information at hand, different kinds of information can be expected to dominate each judgement, so that errors in one judgement need not be consistent with errors in other judgements. In order to evaluate how independent these three judgments are, we examined how adding information that improves one judgement influences the others. Subjects adjusted the size and the global shape of a computer-simulated ellipsoid to match a tennis ball. They then indicated manually where they judged the simulated ball to be. Adding information about distance improved the three judgements in a consistent manner, demonstrating that a considerable part of the errors in all three judgements were due to misestimating the distance. Adding information about shape that is independent of distance improved subjects' judgements of shape, but did not influence the set size or the manually indicated distance. Thus, subjects ignored conflicts between the cues when judging the shape, rather than using the conflicts to improve their estimate of the ellipsoid's distance. We conclude that the judgements are quite independent, in the sense that no attempt is made to attain consistency, but that they do rely on some common measures, such as that of distance.

Cues↗
PubMed1999

Evidence for a mechanism sensitive to the speed of cyclopean form.

We measured Weber fractions for discriminating the speed of cyclopean gratings and Weber fractions for discriminating the speed of luminance gratings. Of our 14 observers, five were unable to see the cyclopean grating sufficiently well to discriminate its speed. One observer experienced great difficulty in discriminating the speed of cyclopean gratings, even though her threshold for detecting cyclopean gratings was low, and even though she discriminated the speed of luminance gratings on the basis of the task-relevent variable. But several observers based their speed discriminations on trial-to-trial variations of the task-relevent variable while ignoring associated trial-to-trial variations in all task-irrelevant variables (specifically: displacement; temporal frequency; spatial frequency; and presentation duration). We conclude that the visual systems of these observers contain a specialized neural mechanism for the speed of cyclopean gratings that supports acute discriminations of speed (Weber fractions were as low as 0.05-0.07).

Differential Threshold↗
PubMed1999

The Poggendorff illusion: a bias in the estimation of the orientation of virtual lines by second-stage filters.

The veridical perception of collinearity between two separated lines is distorted by two parallel lines in the space between them (the Poggendorff illusion). This paper tests the conjecture that the perception of collinearity of separated lines is based on a two-stage mechanism. The first stage encodes the orientation of the virtual line between the proximal terminators of the target lines. The second stage compares this virtual orientation with the orientation of the target lines themselves. Errors can and do arise from either process. Two parallel lines, abutting against the target lines, cause the classical Poggendorff misalignment bias. The magnitude of the bias is increased by Gaussian blur, as is a version of the Poggendorff figure containing only acute angles. In the obtuse-angle figure, on the other hand, blur decreases the misalignment bias. We argue that the acute- and obtuse-angle biases depend upon different mechanisms, and that the obtuse-angle effect is more related to the obtuse-angle version of the Muller-Lyer illusion, which is also decreased by blur. If observers attempt to match the orientation of the virtual line between the two line intersections in the Poggendorff figure they make an error in the same direction as the Poggendorff bias. The orientation of the target lines in the figure, however, is veridically matched to a Gabor-patch probe, unless the target lines are very short, in which case the error is in the same direction as the Poggendorff bias. A small bend in the target lines where they abut the parallels increases the Poggendorff bias if it makes the line more orthogonal to the parallel, but has little effect in the opposite direction. The Poggendorff bias is unlikely to depend upon biases in first-stage linear filters because (a) it still exists in figures composed of short, luminance-balanced lines which are defined by contrast only; and (b) it also exists if the parallels are replaced by grating patches with the same mean luminance as the background. The orientation of the grating in the latter case affects the magnitude of the bias, but even an orientation which should reverse the Poggendorff bias by the mechanism of cross-orientation inhibition fails to do so. The Poggendorff bias is a complex effect arising from several sources. Blurring in second-stage filters with large receptive fields can explain many aspects of the phenomenon.

Contrast Sensitivity↗
PubMed1999

Two processes in stereoscopic apparent motion.

This study investigated the human ability to discriminate the motion direction of sequentially presented depth patterns produced by random-dot stereograms. The stereoscopic (cyclopean) patterns used here consisted of 256 rectangle patches, each of which had an alternative depth position (near or far). Two successive frames of correlated depth patterns made impressions of lateral motion when the pattern position in the second frame shifted laterally. The density of the patches that were near was varied. The Dmax that was measured using the 2AFC method was short when the density was high. The effect of depth reversing in the second frame was also tested. Under low density conditions, the performance was still good against reversing 3-D polarity. However, when the density was high, with depth reversal, motion in the reversed direction was perceived. Reversed motion was observed more often when SOA was small and when the density of near patches was near 1/2. Two strategies seem to exist in stereoscopic motion detecting: a polarity-independent process which matches figures, ignoring their depth polarity, and a polarity-dependent process which operates locally, ignoring 2-D shapes. The latter suggests the existence of a passive process in stereoscopic motion.

Depth Perception↗
PubMed1999

Stereothresholds in persons wtih congenital nystagmus and in normal observers during comparable retinal image motion.

Despite rapid oscillations of the eyes, visual acuity is close to normal in many observers with congenital nystagmus (CN). This study investigated whether binocular hyperacuity thresholds are also close to normal in observers with CN. To do so, we assessed stereothresholds for horizontally and vertically separated line targets in three normal observers and six observers with idiopathic horizontal CN. Stereothresholds in normal observers are better than in the observers with CN, especially for horizontally-separated targets and very small inter-line separations. Stereothresholds remain better in normal observers than in the observers with CN, even in the presence of conjugate retinal image motion simulating that in jerk nystagmus. However, when the simulated CN wave form also includes disconjugate position variability of the foveation periods, normal observers' stereothresholds become similar to those of approximately half of the observers with CN. We conclude that stereothresholds in observers with CN are degraded by the more-or-less constant motion of the retinal image, by excessive vergence instability and, in some observers, by a neural sensory deficit.

Depth Perception↗
PubMed2003

Reduced binocular depth inversion in regular cannabis users.

The binocular depth inversion illusion (BDII) has been shown to be a sensitive measure of impaired visual information processing under conditions including cannabinoid-intoxicated states, alcohol withdrawal, sleep deprivation, and in patients with positive symptoms of schizophrenia. This study assessed whether the BDII could detect subtle cognitive impairment due to regular cannabis use by comparing 10 regular cannabis users and 10 healthy controls from the same community sources, matched for age, sex, and premorbid IQ. Subjects were also compared on measures of executive functioning, memory, and personality. Regular cannabis users were found to have significantly higher BDII scores for inverted images. This was not due to a problem in the primary processing of visual information, as there was no significant difference between the groups for depth perception of normal images. There was no relationship between BDII scores for inverted images and time since last dose, suggesting that the measured impairment of BDII more closely reflected chronic than acute effects of regular cannabis use. There were no significant differences between the groups for other neuropsychological measures of memory or executive function. A positive relationship was found between EPQ-R-psychoticism and cannabis, tobacco, and alcohol use. Cannabis users also used significantly larger amounts of alcohol. However, no relationship was found between BDII scores and drug use other than cannabis or psychoticism. Compared to the other neuropsychological tests used, the BDII appears to be a more sensitive tool for the detection of subtle impairments in visual information processing related to chronic cannabis use.

Adult↗
PubMed2002

Correction of the Pulfrich phenomenon by surgery and laser.

We describe 2 patients who presented with asymmetrical media changes and a spontaneous Pulfrich phenomenon. In the first case, the phenomenon was due to cataract formation and an interocular latency difference of 1.49 ms +/-0.07 (SD) was found. After cataract surgery, the symptoms resolved and no interocular latency difference could be measured. In the second case, the phenomenon was caused by posterior capsule opacification and an interocular latency difference of 1.16 +/- 0.08 ms was found. The symptoms were alleviated and no interocular latency difference could be measured after a neodymium:YAG laser capsulotomy was performed. The symptomatology of the Pulfrich phenomenon is often difficult for patients to explain. Clinicians should be aware of the phenomenon with cataract patients.

Cataract↗